So being attracted to the same sex is supposedly a physical deformation?
Latest research shows most of those who identify themselves
exclusively homosexual have some "variety" in the structure of their brain... that actually make it more like those of the opposite sex. Now, brain structure is definitely
not voluntary, neither is it something that nurture would cause.
Bisexuality cannot be explained with this alone - except that most people are born with it, at least latently. Case in point: people who would otherwise live exclusively heterosexual livess will sometimes adopt "situational homosexuality" in a closed single-sex environment eg. in prisons, oceangoing ships, etc. However, given a free choice in a multi-sex environment, I bet no one knows exactly what defines preference of one sex over the other. Certainly, parental example
cannot be the reason, because children raised by homosexual couples are no more likely to be homosexual or genuinely (as opposed to latently) bisexual than those raised by heterosexual couples. So this would point in the direction of "nature", genes, fetal development or endemic infant development or a combination thereof.
Also, homosexual and bisexual behaviour among animals, both sentient and non-sentient ones, is rather difficult to explain by "choice", since animals tend to be unable to overcome their natural urges. So it would have to be genes and/or something that happens during the growth from embryo to adult, though animals being animals, psychological trauma might not be applicable.
But like we have learned from that other thread, we could just as well blame video games for homosexuality. Or TV and movies. Or those damned subversive comics with their spandex-clad superheroes with secret identities with their deviant superpowers usually manifesting in their teens. Or classic literature. Or dirty cave paintings made by the Neanderthals.